Efficiency and cost-effectiveness of copper electrowinning is and for a long time has been important to the competitiveness of the domestic copper industry. Past research and development efforts in this area have thus focused—at least in part—on mechanisms for decreasing the total energy requirement for copper electrowinning, which directly impacts the cost-effectiveness of the electrowinning process.
In conventional copper electrowinning processes the following reactions occur:
Cathode Reaction:Cu2++SO42−+2e−→Cu0+SO42− (E0=+0.345 V)
Anode Reaction:H2O→½O2+2H++2e− (E0=−1.230 V)
Overall Cell Reaction:Cu2++SO42−+H2O→Cu0+2H++SO42−+½O2  (E0=−0.885 V)
Copper electrowinning according to the above reactions, while desirable in some applications, exhibits several areas of potential improvement for, among other things, improved economics, increased efficiency, and reduced acid mist generation. First, in conventional copper electrowinning, the decomposition of water reaction at the anode produces oxygen (O2) gas. When the liberated oxygen gas bubbles break the surface of the electrolyte bath, they create an acid mist. Reduction or elimination of acid mist is desirable. Second, the decomposition of water anode reaction used in conventional electrowinning contributes significantly to the overall cell voltage via the anode reaction equilibrium potential and the overpotential. The decomposition of water anode reaction exhibits a standard potential of 1.23 Volts (V), which contributes significantly to the total voltage required for conventional copper electrowinning. The typical overall cell voltage is approximately 2.0 V. Decreasing the anode reaction equilibrium potential and/or overpotential can reduce cell voltage, and thus conserve energy by decreasing the total operating costs of the electrowinning operation.
One way that has been found to potentially reduce the energy requirement for copper electrowinning is to use alternative anode reaction electrowinning (i.e., oxidation of ferrous ion to ferric ion at the anode), which occurs by the following reactions:
Cathode Reaction:Cu2++SO42−+2e−→Cu0+SO42− (E0=+0.345 V)
Anode Reaction:2Fe2+→2Fe3++2e− (E0=−0.770 V)
Overall Cell Reaction:Cu2++SO42−+2Fe2+→Cu0+2Fe3++SO42− (E0=−0.425 V)
The ferric iron generated at the anode as a result of this overall cell reaction can be reduced back to ferrous iron (i.e., “regenerated”) using sulfur dioxide, as follows:
Solution Reaction:2Fe3++SO2+2H2O→2Fe2++4H++SO42−
The use of the ferrous/ferric anode reaction in copper electrowinning cells lowers the energy consumption of those cells as compared to conventional copper electrowinning cells that employ the decomposition of water anode reaction, since the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) occurs at a lower voltage than does the decomposition of water.
Conventional copper electrowinning processes produce solid copper cathode sheets. Copper powder, however, is an alternative to solid copper cathode sheets. Production of copper powder as compared to copper cathode sheets can be advantageous in a number of ways. For example, it is potentially easier to remove and handle copper powder from an electrowinning cell, as opposed to handling relatively heavy and bulky copper cathode sheets.
In traditional electrowinning operations yielding copper cathode sheets, harvesting typically occurs every five to eight days, depending upon the operating parameters of the electrowinning apparatus. Copper powder production has the potential, however, of being a continuous or semi-continuous process, so harvesting may be performed on a substantially continuous basis, therefore reducing the amount of “work-in-process” inventory as compared to conventional copper cathode production facilities. Also, there is potential for operating copper electrowinning processes at higher current densities when producing copper powder than with conventional electrowinning processes that produce copper cathode sheets, capital costs for the electrowinning cell equipment may be less on a per unit of production basis, and it also may be possible to lower operating costs with such processes. It is also possible to electrowin copper effectively from solutions containing lower concentrations of copper than using conventional electrowinning at acceptable efficiencies. Moreover, copper powder exhibits superior melting characteristics over copper cathode sheets and copper powder may be used in a wider variety of products and applications than can conventional copper cathode sheets. For example, it may be possible to directly form rods, shapes, and other copper and copper alloy products from copper powder. Copper powder can also be melted directly or briquetted prior to melting and conventional rod production.
Although, in general, the use of the ferrous/ferric anode reaction in connection with copper electrowinning is known, use of the ferrous/ferric anode reaction in connection with the electrowinning of copper powder is not known. What is needed is an effective and efficient process for producing a high-quality, saleable copper powder product that exhibits the advantages of alternative anode reaction electrowinning chemistry.